(1) Field of the Invention
The present invention relates to a base film for a magnetic recording medium. More particularly, it relates to a base film suitable for a super high density magnetic recording medium.
(2) Description of the Prior Art
With an increase of the recording density in the magnetic recording medium, the surface smoothness required for a polyester film used for the recording medium is increased. On the other hand, in order to improve the operation adaptability at the film-forming step or the magnetic tape-preparing step, slip characteristics are required for the surface of the film. Moreover, the slipness of the magnetic tape is crucial. Namely, it is required for a polyester base film suitable for a magnetic recording medium to have excellent smoothness and good slip characteristics.
As means for satisfying these requirements, there has been adopted a method in which fine particles are incorporated in the starting polymeric material for formation of a smooth film. However, according to this conventional technique, if one of the smoothness and the slip characteristics is improved, the other property is degraded, and it has heretofore been impossible to improve both the properties simultaneously.
A conventional magnetic recording tape is of the so-called coating type, which is prepared by coating a base film with a magnetic paint comprising a magnetic powder and an organic polymer binder, and the coating layer has a relatively large thickness, i.e., ordinarily 3 to 6.mu.. Accordingly, it seldom happens that the convexities and concavities on the surface of the base film are reproduced in the magnetic layer to cause drastic reduction of the electromagnetic performances. Therefore, in the conventional base film for the magnetic recording medium, a method has been employed wherein finely divided particles are incorporated in the starting polymeric material or inner particles are formed in the starting polymeric material from the polymerization catalyst residue, whereby slip characteristics are imparted to the film.
However, this magnetic recording medium of the coating type does not satisfy the recent requirement of realizing a super high magnetic recording density. Accordingly, a so-called metallized film tape having a highly magnetic, thin metal surface layer formed by vacuum evaporation or sputtering without the use of a binder has become popular as a super high recording density video tape instead of the magnetic recording medium of the coating type.
The thickness of the magnetic metal layer of the above-mentioned metallized film tape is usually 0.05 to 0.2.mu., namely, much thinner than that in the magnetic tape of the coating type. Therefore, the surface configuration of the base film used is completely reproduced on the surface of the metallized film tape. Accordingly, if the surface roughness of the base film is large, the electromagnetic performances of the metal thin-film video tape is extremely reduced. In fact, even if a metal thin-film video tape formed from a finely divided particle-incorporated base film is used in a VTR (video tape recorder), no image is reproduced, namely, this tape cannot be used as a video tape at all.
In view of the electromagnetic performances, a base film having quite a smooth surface is preferred, but practically, the following problem arises with such a smooth surface film. More specifically, a vacuum-deposited metal film formed on a base film having quite a smooth surface is inferior in slip characteristics of the surface, and therefore, unless, for example, a protecting film layer is formed thereon, many scratches are inevitably produced on the surface of the vacuum-deposited metal film by contact with various guide pins, heads and the like in the tape-forming process after the vacuum evaporation step or during the use thereof in a video tape recorder.
As a base film for the high density magnetic recording medium in which the foregoing defect is eliminated, reduction of the electromagnetic performances is minimized and the slip characteristics are improved, we previously proposed in U.S. Pat. No. 4,233,352 a polyester film having on at least one surface thereof a discontinuous film formed of worm-like nodules composed of a mixture of a water-soluble polymer and a polysiloxane or styrene-butadiene rubber. The above-mentioned problem has been substantially solved by this proposal and a good image can be reproduced even when a metallized film is used as a video tape.
We furthered our researches on base films of this type, and found that another problem arises at the step of forming a metallized film video tape. Namely, it was found that by abrupt elevation of the temperature of the base film, oligomers ooze out from the interior of the polyester film and are precipitated on the film surface. When the oligomers are precipitated, the surface smoothness is degraded by the solid particles of the oligomers and bad influences are imposed on the electromagnetic performances of the metallized film tape. Moreover, when the size of the oligomer particles is too large, it is impossible to perform the vacuum deposition and such defects as pinholes are formed, and there is a risk of exposure of the oligomer particles on the surface of the metallized film. The resulting magnetic tape is liable to exhibit undesirably increased drop-out number. Moreover, accordingly, after formation of a magnetic tape, a problem of drop-out arises. Moreover, the slip characteristics and abrasion resistance of the film tend to be readily varied in a broad temperature range, and if the film is exposed to a high temperature at the vacuum deposition step or the like, the slip characteristics are drastically reduced, and the strength and abrasion resistance of the film are reduced.